The U.S. Environmental Protection Agency this week reported a 1.6 percent decrease in the nation’s greenhouse gas emissions from 2010 to 2011, but levels remain 8 percent higher than in 1990.

The decrease can be attributed to “reduced emissions from electricity generation, improvements in fuel efficiency in vehicles with reduction in miles traveled and year-to-year changes in the prevailing weather,” the EPA's "Inventory of U.S. Greenhouse Gas Emissions."

Although 2011 emissions declined, the U.S. continues to be the largest contributor to greenhouse gas emissions in the world to date according to the World Resources Institute.

“Greenhouse gases trap heat and make the planet warmer. Human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere over the last 150 years,” said the EPA.

The decrease was “primarily due to a decrease in carbon intensity of fuels consumed to generate electricity.”

Total emissions for 2011 were measured at 6,702 million metric tons of carbon dioxide. Most greenhouse gas emissions are generated from the production of electricity with the burning of fossil fuels - mainly coal and natural gas - along with transportation and industry.

“The largest source of emissions is energy and that was not a surprise,” said Jennifer McGraw, climate change researcher for the Chicago nonprofit Center for Neighborhood Technology. “The EPA is showing emissions declining in part because the market for coal has declined."

According to the EPA, emissions increase and decrease year to year and can be based on changes in the economy. The 2011 drop in gas emissions was related to a decrease in the consumption of coal, while natural gas and hydropower use increased.

But the EPA’s national inventory is not comparable to a city level, said McGraw.

Chicago and other major metro areas are leading contributors to U.S. greenhouse gas emission levels. The city emits more than 34 million metric tons of carbon dioxide in greenhouse gases, with 12.4 metric tons per household, the Chicago-based Center for Neighborhood Technology reported in 2010.

Chicago’s per capita emissions exceeds major cities such as New York City and London. Chicago had the fourth highest emission level. Denver lead with 21.1 metric tons per person, followed by Houston at 14.1 and Los Angeles at 13 metric tons.

“There are systematic changes happening partly because of policy changes and partly because of changes in the market economy,” said McGraw.

“A lot of the conversation has been about national policy. That’s important, however local communities are making it too. There is a lot more that can be done on the local level. Other cities have different challenges than Chicago. They have different energy profiles.”

The center projects Chicago’s emissions will increase 11 percent by the year 2050.

“Chicago is very hot and it’s very cold, there is energy needed to make life livable and comfortable,” said McGraw. “But we need to figure out ways to make that more sustainable on a local scale.”